In the oil and gas industry, underground formations are probed and characterized by well logging tools. Acoustical properties of a formation, such as compressional (P) and shear (S) wave speeds, are measured with sonic tools and have applications in seismic correlation, petrophysics, rock mechanics and other areas. Traditionally, these measurements are conducted after or in between drilling and are called wireline logging. In wireline logging, sonic monopole tools are used to measure P in all formations and S in fast formations; sonic dipole tools are used to measure S in both fast and slow formations (Kurkjian and Chang, 1986). Exxon reported experimental work on the concept of wireline quadrupole tools (Chen 1989, Chen and Eriksen 1991, Winbow etc. 1991). In the last decade there has been a trend towards measuring the rock properties while drilling is taking place, that is Logging While Drilling or LWD. Schlumberger's ISONIC tool is an example of such LWD tools. The ISONIC is a monopole sonic tool. Other companies in the industry have commercialized combined monopole and dipole LWD tools. For all sonic tools, tool borne energy, which contains no information about the formation, is sought to be removed or attenuated. For wireline tools, the tool waves are reduced with designs such as slotted sleeves, isolation joints and flexible tool structures. For LWD tools, tool waves carried by the prominent and stiff tool body, which is essentially the drill collar with sensors and electronics mounted on it, is a more serious challenge; hence deriving formation P and S speeds from LWD data is not nearly as straightforward as in wireline.
A sonic tool has usually one or more sources and multiple receivers. There are multiple paths for the acoustic energy emitted by the source to reach the receivers. The part of acoustic energy propagating through the formation and mud provides useful information for characterizing the formation. The part of acoustic energy propagating through the tool body does not contain useful information and is sought to be removed.
A monopole logging tool, wireline or LWD, employs single or multiple monopole acoustic source(s) as well as receivers; they transmit and detect acoustic energy uniformly in all azimuthal directions in the plane perpendicular to the tool axis. It is well understood based on wave propagation theory that a monopole tool can excite and detect P and Stoneley waves in all formations, regardless of formation speed. In addition, a monopole tool is capable of generating and detecting S waves in fast formations where the formation shear speed is faster than the sound speed in the mud. In wireline monopole tools, the tool borne energy is delayed and suppressed by techniques such as slotted receiver housings and/or damping. In LWD monopole tools, dealing with the tool waves is a more difficult issue. Schlumgerger's ISONIC tool achieves tool wave attenuation over a selected frequency band with a specially designed periodic array of grooves machined on the collar section between the transmitter and receivers. This technique is disclosed in U.S. Pat. No. 5,852,587 to Kostek et al., issued Dec. 22, 1998.
A dipole tool generates and receives the flexural mode in a borehole surrounded by a formation. The term “Dipole” refers to the azimuthal profile cose for the transmitter, receivers and the acoustic field associated with the flexural mode. The flexural mode propagation asymptotes to the formation shear speed at the low frequencies, and to the mud-formation interface wave speed at high frequencies. Thus S wave speed of the formation can be derived from the measured flexural mode (Kurkjian and Chang 1986). Wireline dipole tools use flexible (i.e. acoustically slow) or acoustically transparent housings for receivers to minimize or avoid tool effects on the measured borehole flexural mode, in other words, to approximate the fluid-filled (no tool) borehole condition. These tools are also designed with some form of acoustic isolator or attenuator between the source and receivers to reduce the excitation of tool waves.
Applying the wireline dipole shear technique to LWD encounters serious challenge of tool wave interference. LWD tools can not be made very flexible or slow as wireline tools because of tough drilling conditions in which LWD tools need to operate. The body of an LWD tool, essentially a drill collar, provides a propagation path of acoustic energy between the acoustic source and receivers. Both borehole flexural and tool flexural waves are present in similar frequency and slowness ranges and thus can not be easily separated. In addition, the presence of the tool alters the borehole mode from the no tool condition; the alteration is the most substantial under conditions where the borehole and tool flexural waves are close in speed (Hsu and Sinha 1998, Rao and Vandiver 1999). These two coexisting borehole and tool flexural modes can be predicted by modeling. However, to extract formation shear from these altered modes is much more difficult and less robust than extracting shear in the wireline situation where only the borehole flexural mode is present. One typical approach to alleviate the problem is to attenuate the tool borne energy with attenuators placed in the drill collar in between the transmitter and receivers (Birchak etc 1996, Molz and Leggett 2000). However, even if the tool borne energy is substantially attenuated and becomes weaker than the borehole flexural mode signal, the resultant dispersion of the borehole flexural mode will typically not asymptote to the formation shear speed. This can significantly reduce the accuracy of extracting formation shear from the data.
Borehole quadrupole mode has similar dispersion characteristics as that of the borehole dipole mode, with the quadrupole dispersion taking place at higher frequencies relative to the dipole mode. Quadrupole refers to the azimuthal profile cos 2θ of transmitter, receivers, and acoustic field associated with the quadrupole modes. Both dipole and quadrupole modes asymptote to the fluid-formation interface wave speed at high frequencies and to the formation shear speed at low frequencies. More specifically, the quadrupole mode actually crosses the shear speed; however, there is practically no energy at frequencies below the crossing point. Exxon reported on a wireline quadrupole experimental tool for S wave measurement (Chen 1989, Chen and Eriksen 1991, Winbow etc. 1991). This tool is disclosed in U.S. Pat. No. 4,932,003 to Winbow et al., issued Jun. 5, 1990, and in U.S. Pat. No. 5,027,331 to Winbow et al., issued Jun. 25, 1991. In the Exxon experimental quadrupole tool, an acoustic isolator between source and receivers was mentioned, and no consideration of tool effect was reported.